Technical Field
The present invention relates to the field of environmental protection and comprehensive utilization of resource, particularly to a method and a system of treating biomass wastes by biochemistry-thermochemistry multi-point interconnection.
Description of Related Art
Biomass wastes include wastes produced by processing and consumption of a variety of biomass, such as the biodegradable part of municipal solid wastes like fruit and vegetable peels from kitchen and food residues, agricultural wastes like livestock manure and crop straws, industrial wastes like slaughterhouse wastes and brewing residues, and municipal sludge, etc. Biomass wastes are an important renewable source. Anaerobic digestion is a traditional biochemical technology to recycle biomass wastes. With anaerobic digestion technology, biomass wastes can be converted into green energy gas-methane, both producing energy source and reducing greenhouse gas emission, and digestion residues (including slurry and fiber digestate) containing nutrients such as nitrogen phosphorus and humus needed by plants can be used as fertilizer or soil ameliorant. As a matter of fact, however, treating biomass wastes with anaerobic digestion technology has a benefit much lower than expected, lying in: 1) organic acids and ammonia of high concentration are produced when easily biodegradable wastes (e.g., fruit and vegetable peels from kitchen, livestock manure) are being degraded. These metabolites inhibit the anaerobic digestion process when accumulated in the anaerobic digestion reactor, resulting in less efficient, unstable, or even failed anaerobic digestion, which needs a very long time to recover. 2) The anaerobic degrading rate of recalcitrantly biodegradable wastes (e.g., crop straws and ground litter with abundant lignocellulose) is relatively slow because of their high content of lignin and cellulose, and they need to be retained in the anaerobic reactor for a very long time. 3) Residues of anaerobic digestion need to be separated into slurry and fiber digestate by solid-liquid separation before further use. However, the solid-liquid separation is not efficient, wherein the solid content of the fiber digestate is only about 8%-25%, while the solid content of the slurry is often 3%-5%. This means if the slurry is drained to the municipal sewer system rather than applied to land directly, it needs to be further treated, and the treating cost of the slurry is quite high. 4) There are a large amount of degradable organic substances left in the fiber digestate due to uncompleted anaerobic digestion, making the digestate unstable, perishable, easy to turn rancid and have leachate, which needs to be further composted before land application. 5) A traditional way to use slurry and fiber digestate is applying them to earth, but, as organic fertilizer, their efficiency is not significant than chemical fertilizers in short term. Thus, they are much less competitive than the chemical fertilizers in the market. Moreover, the application of fertilizer is influenced by the planting season of crops, so a large space is needed to store slurry and fiber digestate. Particularly for biomass wastes produced in urban areas, slurry and fiber digestate produced after anaerobic digestion have to be transferred to places like suburb and countryside by long-distance transportation, which largely increases the cost and difficulty of transportation. Therefore, for sustainable development and promotion of biomass waste anaerobic digestion technology, it is desired to increase the efficiency and stability of the anaerobic digestion process, and to improve the dewatering performance, biochemical stability, storability, transportability and the like of the digestate.
Existing biochemistry-thermochemistry combined technology has only single-point and one-way connection, for example, adding charcoal (a kind of biochar) into an anaerobic digestion reactor according to a Chinese invention patent “Method using charcoal for promoting anaerobic digestion of sludge to produce methane” (Application number: 201310175120.0). Charcoal is added during the composting process as described in a Chinese invention patent “Use of charcoal as exogenous conditioning agent in sludge composting” (Application number: 201310077278.4). As described in a document “A new concept for enhancing energy recovery from agricultural residues by coupling anaerobic digestion and pyrolysis process” (F. Monlau et. al, Applied Energy, 148 (2015) 32-38), heat produced by methane of an anaerobic digestion unit is used for drying fiber digestates, and the dried digestates are used as the feed of a pyrolyzing unit.